FIG. 1 schematically shows the anatomy of a normal human ear. The ear typically transmits sounds, such as speech sounds, through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes three chambers along its length, an upper chamber known as the scala vestibuli, a middle chamber known as the scala media, and a lower chamber known as the scala tympani. The cochlea 104 forms an upright spiraling cone with a center called the modiolus where the axons of the auditory nerve 105 reside. These axons project in one direction to the cochlear nucleus in the brainstem and they project in the other direction to the spiral ganglion cells and neural processes peripheral to the cells in the cochlea. In response to received sounds transmitted by the middle ear 103, sensory hair cells in the cochlea 104 function as transducers to convert mechanical motion and energy into electrical discharges in the auditory nerve 105. These discharges are conveyed to the cochlear nucleus and patterns of induced neural activity in the nucleus are then conveyed to other structures in the brain for further auditory processing and perception.
Hearing is impaired when there are problems in the ability to transmit sound from the external ear to the inner ear, or there are problems in the transducer function within the inner ear. To improve impaired hearing, there are several types of auditory prostheses that have been developed, such as middle ear and inner ear implants, that can restore a sense of partial or full hearing. For example, when the impairment is related to the operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic stimulation to the auditory system in the form of amplified sound. When the impairment is associated with the transducer function in the cochlea 104, a cochlear implant system may be used. Cochlear implant systems can restore some sense of hearing by direct electrical stimulation of the neural tissue of the cochlea. The cochlear implants typically include an implanted electrode 106 having an electrode lead 108 and an electrode array 110. The electrode array 110, which is threaded into the cochlea 104 through an opening in the round window or a cochleostomy site 112, usually includes multiple electrode contacts 114 on its surface that electrically stimulate auditory nerve tissue with small currents delivered by the contacts 114 distributed along the electrode array 110. These electrode contacts 114 are typically located toward the distal end or apical portion of the electrode 106 and are in electrical communication with an electronics module (not shown) that produces an electrical stimulation signal for the implanted electrode contacts 114 to stimulate the cochlea 104. The electrode lead 108 includes the proximal end or basal portion of the implanted electrode 106 that typically goes from the electronics module (e.g., implanted or externally mounted) to the electrode array 110. The electrode lead 108 usually has no electrode contacts, except perhaps a contact that acts as a ground electrode, and encloses the wires that connect to and deliver the electrical stimulation signals to the electrode contacts 114 on the electrode array 110.
In cochlear implant surgery, one surgical technique is a facial recess approach to the middle ear, which involves removing considerable amount of bone in the mastoid region to reach the middle ear region, which is then followed by the opening of the scala tympani either by round window entrance or cochleostomy and the insertion of the electrode array into the cochlea. The facial recess approach is only attempted by experienced surgeons. A recent advancement in this surgical technique involves a minimally invasive procedure in which robot assisted narrow drilling is done in the mastoid bone to reach the middle ear region. The drilling forms a narrow key hole in the mastoid bone, and the electrode is inserted through this key hole. However, there are difficulties with inserting the electrode through this highly porous mastoid bone due to the small size of the hole and also with the subsequent positioning and securing the electrode lead after insertion.